I had reason to remove the tank on the Darmah so disconnected the fuel lines from the tap ends. Re-assembled and when I went out the next day noticed that no fuel flowed into the front carb fuel line even when tickled. Hmmm.... checked that fuel was running out of the tap so disconnected the float bowl and fuel was reaching inside the carb and the float cut the supply off. Reconnected the float bowl and hey presto all was working fine. Fuel flowed when the carb was tickled and stopped when the bowl was full. Bike started fine and ran splendidly for 30 miles or so. All I could think it was, was some sort of weird air/vacuum lock
While maybe not relevant to your current issue, I have a couple of lovely quality Dellorto manuals that are wasting away in my drawer; "A Guide 1.1" and "The Carburetor Manual" if you could make use of them ??
Almost certainly an airlock, although unusual. We take gravity for granted a bit don't we? Have had to prime the feedline to a mechanical fuel pump many times over the years as a last resort to getting it to "pump"
Thanks for the confirmation... it's so unusual that it's the first I've seen of it in 40 years of owning the bike. So where is the air trapped then, in the float bowl it self? And would opening the throttle a couple of times and/or spinning the motor for a second help clear it? The front pot fuel line has always been a little strange with it never ever been 'full' of fuel even when running. Here's a couple of photos showing the level when the fuel tap is first turned on and the level after been tickled.
The lie of the fuel line might have something to do with it. Does it have continuos downward run to the carb or is there a horizontal section possibly with a slightly upward direction?
You might be on to summat here. A few months back I replaced the fuel line for a slightly longer one as the original had wear marks in it from where it was rubbing against the (pod) air filter. And the 1st photo below shows there is a slight upturn where it meets the carb. Although I'm not sure if that was because I was faffing about with it when trying to clear the lock or it has been like that since I fitted the replacement line. There's not much of drop to work with between the tap & the carb inlet, say around 2" or so, and if the line is too short & direct then it'll rub against the filter. Here's a couple of photos, the first showing how it was and the second after I had tweaked it a little where I did see tiny bubbles coming from the carb inlet (and a burp from the tank).
hard to say definitively as the moment has passed but route is everything sometimes as said. Float may have decided to stay "up" despite bowl being empty, maybe even dry needle valve related. sometimes a careful "tap" on the float chamber will make things behave. I had to do this recently on an E-Type on the motorway, and luckily it worked. At least these are still lovely and simple to cure compared to today's electronic injection systems.
Well, I reckon progress has been made. The air lock happened again so I investigated further and there was nothing up with the carb or line or indeed the tap which I disassembled and removed from the tank. In my faffing about I did notice one oddity when there was a small amount of fuel left in the carb end of the line, with a large air gap above it, that didn't drain out even when it was removed from the carb... So I'm thinking the air pressure outside the line was sufficient to overcome the air pressure (vacuum perhaps) above the fuel. So bearing in mind Derek's suggestion above I splayed the tap slightly outwards which allowed a shorter & straighter line to be run that didn't run against the pod filter. When I first connected it all up the fuel line was completely full with fuel but when left for a few days with the tap switched off the air gap appears, due to I'm assuming, evaporation in the carb bowl. But I've had no more air locks even when the line had no fuel in it at all. Having said all that I'm not sure why it doesn't happen on the rear carb... Anyway, here's some photos: Here's the new routing of the line with no air gap & full of fuel. Here's the line after a few days left in the garage with the tap off.
